1. Field of the Invention
The present invention relates generally to optical devices such as lasers, and fiber optic data transmission systems employing the same, and particularly to a novel wavelength-locked loop servo-control circuit for optimizing performance of semiconductor optical amplifiers.
2. Description of the Prior Art
Wavelength Division Multiplexing (WDM) and Dense Wavelength Division Multiplexing (DWDM) are light-wave application technologies that enable multiple wavelengths (colors of light) to be paralleled into the same optical fiber with each wavelength potentially assigned its own data diagnostics. Currently, WDM and DWDM products combine many different data links over a single pair of optical fibers by re-modulating the data onto a set of lasers, which are tuned to a very specific wavelength (within 0.8 nm tolerance, following industry standards). On current products, up to 32 wavelengths of light can be combined over a single fiber link with more wavelengths contemplated for future applications. The wavelengths are combined by passing light through a series of thin film interference filters, which consist of multi-layer coatings on a glass substrate, pigtailed with optical fibers. The filters combine multiple wavelengths into a single fiber path, and also separate them again at the far end of the multiplexed link. Filters may also be used at intermediate points to add or drop wavelength channels from the optical network.
Optical communication links in systems employing WDM or, optical networks in general, require amplification to extend their distances. For example, optical signal amplification are needed in optical links for applications such as disaster recovery in a storage area network or parallel sysplex. There are many types of amplifiers, however, for some wavelength ranges of interest, semiconductor optical amplifier devices (SOAs) have emerged as being extremely useful. An SOA functions much like an in-line semiconductor laser diode in that it is optically pumped for amplifying incoming optical signals without requiring optical/electrical conversions. However, the SOA also broadens the optical spectrum of the amplified light, which may induce undesired effects such as dispersion and modal noise that limit the effectiveness of this technology.
Particularly, as illustrated in FIG. 1, the basic SOA device 100 (also known as a semiconductor laser amplifier or xe2x80x9cSLAxe2x80x9d) is very similar in construction to a Fabry Perot semiconductor laser diode, comprising semiconductive layers 110, 111 and an active layer 112 forming an optical cavity which receives an input optical signal 120. Generally, when an electrical current 115 is pumped through the device, electrons are excited in the optical cavity 112 to effect gain of the input signal 120 in the direction of propagation. The output optical signal 130 is thus an amplified version of the input signal. It is understood that mirrors may be implemented in the optical cavity for increasing the effective path length through the gain medium, and hence increase the overall gain. The SOA offers potential advantages over other optical amplification technologies such as doped fiber amplifiers. In particular, the SOA can be monolithically integrated with other semiconductor devices on a common chip or substrate, e.g., GaAs or hybrid Si on insulator, and mass produced at low cost. SOAs can easily amplify light at various wavelengths, including 1300 nm and 850 nm which is a unique feature, since erbium doped fiber amplifiers (EDFAs) operate only at wavelengths near 1550 nm, and more exotic doped fiber amplifiers at other wavelengths are more expensive and difficult to manufacture. This is an important advantage, as the SOA is a low cost solution to amplify the 1300 nm and 850 nm windows most commonly used in data communication systems such as ESCON, Fibre Channel, and Gigabit Ethernet. The SOA is also a very compact and highly reliable device. However, an SOA differs from a laser diode in that the SOA operates below the threshold current required for laser action. (In a variant design, the traveling wave SOA, may be operated above threshold but has other design and manufacturing problems which have so far prevented its becoming a commercially available device). Due to this, the light emerging from an SOA has a very broad spectral width, around 20-50 nm and, in some cases, several hundred nanometers, as opposed to a typical narrowband laser which has about 2-3 nm spectral width. Thus, an optical signal entering the SOA will be amplified, but suffers a significant spectral broadening; the additional optical power is spread across a much wider frequency range. Not only is this an inefficient way to amplify the light, but the spectral broadening causes secondary effects such as increased dispersion, modal noise, and mode partition noise on the communication link; these noise sources can exhibit a noise floor, which means that the noise limits the maximum link distance regardless of the strength of the amplified signal. For this reason, SOAs have not been widely deployed in very long distance links, although they have found applications in shorter data and telecommunication systems.
Furthermore, if the SOA is operated at higher voltages or currents (still below threshold), the gain increases and the spectral broadening becomes worse. In principle, the SOA output may be optically filtered with a narrow band element such as an array waveguide grating or multilayer thin film interference filter, as these devices can be integrated onto the semiconductor substrate. However, such filters are very difficult to fabricate with their center wavelength exactly aligned to the peak of the SOA output spectrum, hence they have unacceptably high insertion loss (up to several dB) which cancels out the gain of the optical amplifier. Further complicating the problem, the SOA tends to have a high insertion loss, as well as high spontaneous emission noise due to random generation of photons at the amplified wavelengths. The SOA spectrum also drifts with changes in temperature or bias voltage, as well as with the aging of the SOA diode.
It would thus be highly desirable to provide a system and method for automatically compensating for the undesirable effects of an SOA, and particularly a system and method for overcoming the spectral broadening associated with SOA devices.
It would thus be highly desirable to provide a servo-control feedback loop for stabilizing the SOA output and tracking the center wavelength of the amplified signal to the peak of an optical filter passband with high accuracy to enable higher gains than currently achievable with SOAs.
It is therefore an object of the present invention to provide a system and method for overcoming the spectral broadening associated with semiconductor optical amplifier (SOA) devices.
It is another object of the present invention to provide a servo-control loop for implementation in an SOA device that enables for dynamic tracking of the center wavelength of the amplified signal to the peak of an optical filter passband with high accuracy.
It is a further object of the present invention to provide a servo-control loop for implementation in an SOA device that provides stabilization of the SOA output and provides tracking of the center wavelength of the amplified signal to the peak of an optical filter passband to enable higher gains than currently achievable with SOAs.
It is another object of the present invention to provide a servo-control loop for implementation in an SOA device that is implemented on a common semiconductor substrate and thus may be integrated with the SOA diode design.
It is still another object of the present invention to provide a servo/feedback loop, referred to as a xe2x80x9cwavelength-locked loop,xe2x80x9d that provides stabilization of the SOA output and provides tracking of the center wavelength of the amplified signal to the peak of an optical filter passband to enable higher gains, thereby enabling significantly larger link power budgets and longer supported distances in fiber optic data communication systems.
It is yet still another object of the present invention to provide a servo/feedback loop, referred to as a xe2x80x9cwavelength-locked loop,xe2x80x9d in an optical transmission system, that may be implemented for modulating the amplitude, phase, and/or frequency of an optical signal.
It is a further object of the present invention to provide a servo/feedback loop, referred to as a xe2x80x9cwavelength-locked loop,xe2x80x9d in an optical transmission system, that enables application of several different types of modulation to an optical signal, including digital data modulation, analog modulation, and may be used for analog-to-digital conversion, or digital-to-analog conversion applications.
It is still a further object of the present invention to provide a servo/feedback loop, referred to as a xe2x80x9cwavelength-locked loop,xe2x80x9d in an optical transmission system, that provides for binary modulation of an optical signal in addition to multi-level signaling.
Thus, according to one aspect of the invention, there is provided a system and method for improving gain efficiency in a semiconductor optical amplifier, the method comprising steps of: receiving an input optical signal to be amplified; providing a bias signal for input to a semiconductor optical amplifier and generating an amplified output optical signal from the input optical signal having a peaked spectrum function including a center wavelength according to an input bias signal value; providing an optical filter element for passing output optical signals amplified by the semiconductor optical amplifier device, the optical filter element exhibiting a peaked passband function including a center wavelength; and, providing automatic real-time mutual alignment of the center wavelength of the amplified optical signal output with the optical filter having the peaked passband function so that the output optical signal is maximally transferred through the optical filter element, thereby resulting in increased semiconductor optical amplifier gain
According to another aspect of the invention, there is provided a system and method for modulating an optical signal characterized as a peaked spectrum function having a center wavelength and employed in an optical system including a wavelength selective device implementing a peaked passband function having a center wavelength for passing the optical signal, the method comprising the steps of: applying a dither modulation signal at a dither modulation frequency to the optical signal, and inputting the dither modulated optical signal to the wavelength selective device; converting a portion of the dither modulated optical signal to a feedback signal; continuously comparing a frequency characteristic of the feedback signal with the dither modulation signal and generating an error signal representing a current offset amount between the peaked spectrum function center wavelength of the optical signal and the peaked passband function center wavelength of the wavelength selective device, the current offset amount indicating a degree of modulation of the optical signal communicated in the system; receiving a data information signal to be communicated in the optical system and comparing a difference between a desired offset amount associated with each data information signal to be communicated and the current offset amount; dynamically adjusting the center wavelength of the optical signal according to the offset difference for achieving the desired offset amount between the peaked spectrum function center wavelength of the optical signal and the peaked passband function center wavelength of the wavelength selective device, wherein the optical signal is modulated according to the desired offset amount.
Advantageously, the system and methods of the present invention enables the widespread application of SOAs, and, optical amplifiers in general, in data communication systems. Furthermore, all the components of the feedback loop may easily be fabricated on a common semiconductor substrate using standard photolithographic methods, and thus may be integrated with the SOA diode design. Further, the implementation of the wavelength-locked loop for providing modulation of optical signals according to the invention, is advantageous for application areas such as radar and sonar signal processing, image sampling and transmission, holographic storage, and other areas which today exploit externally modulated optical generators, e.g., laser diodes.